1. Field of the Invention
The present invention relates to a coating applied to powdered nuclear fuel materials prior to sintering.
2. Background of the Invention
The inside of a nuclear reactor is an extremely harsh environment. Temperatures can be as high as 1800° C. at the center of the fuel pellet, and all of the components are exposed to highly corrosive steam.
Neutrons cause fission reactions in typical Light Water Reactors (LWRs). LWRs utilize the energy produced from fission reactions to heat water or steam in the reactor core. This water or steam travels through a heat exchanger to heat clean water into clean steam, and this clean steam turns downstream turbines to produce mechanical energy or motion. The mechanical energy turns a generator which results in the production of electricity.
A representation of a typical LWR core is depicted in FIG. 1A. A nuclear reactor core 20 contains a series of co-planarly arranged fuel rods 24 between which are positioned control rods 22. The control rods 22 are made of highly neutron absorbent materials such as silver, indium, hafnium, boron, and cadmium. Depending on power requirements called for by the grid, the control rods 22 are either partially or fully inserted or removed to moderate the flux of neutrons, and therefore the amount of fission. This moderation is proportional to the amount of energy produced.
FIG. 1B depicts a detail view of the fuel rods 24, which contain the fissile material, typically in the form of fuel pellets 26. Surrounding the fuel pellets 26 is a cladding layer 28, which is typically made of zirconium or a zirconium alloy. As can be seen in FIG. 1C, a gap 30 between the fuel pellets 26 and cladding 28 must be provided to allow for the expansion of the fuel pellet 26 and the cladding 28. Expansion occurs because of the nuclear irradiation and fuel swelling as a result of the production of fission gases.
Typical fuel pellets are made of sintered uranium dioxide (UO2). The uranium present in the pellets is mostly uranium-238, which has been enriched to contain approximately three percent uranium-235. The uranium-235 is the major fuel of the LWR, but the uranium-238 is fissionable and produces plutonium-239, which also fuels the LWR. In some reactors, the pellets are made of both uranium and plutonium oxides and are referred to as mixed oxide fuels.
One particular problem facing state of the art fuel pellets 26 is the heat conduction from the center of the pellet 26 to the exterior of the cladding 28. The heat conduction of uranium dioxide is poor relative to that of the cladding material. This can cause high heat build-up within a fuel rod, leading to failure. Temperatures at the center of the pellet can reach 1800° C. inasmuch as the heat has nowhere to dissipate. This is because heat conduction of uranium dioxide is 5 W/mK (where mK is meters-Kelvin), while the heat conduction of its surrounding, yet physically remote zirconium cladding is 35 W/mK. The poor heat conduction is exacerbated by the expansion gap 30 between the pellet 26 and the cladding 28.
Several other fuel forms exist besides uranium dioxide, such as uranium nitride (UN), uranium carbide (UC), uranium-zirconium hydrides, and uranium-molybdenum alloys, among others. These fuel forms have significant advantages over uranium dioxide in terms of the uranium density in the pellet, heat conduction. The invention improves fission gas control, fuel/cladding interactions and high temperature behavior. Despite the excellent properties of these metallic fuels, they have yet to see any great market penetration because of their inferior corrosion resistance.
In loss-of-coolant-accidents (LOCA), the corrosion resistance of all the reactor components is crucial because any water remaining in the vessel will heat up and turn to high-pressure steam. This high-pressure steam will oxidize many of the reactor core components, including the fuel pellets. State of the art uranium dioxide fuel pellets are resistant to oxidation because they are already oxidized. However, oxidation of the alternative fuel forms mentioned will create hydrogen gas, which is likely to result in an explosion. Hydrogen explosions can be devastating to reactor containment as evidenced by the disaster in 2011 in Fukushima, Japan in which a hydrogen explosion literally blew the roof off of a containment building.
In order to increase safety in the event of a LOCA, uranium dioxide fuel pellets are predominantly used at the expense of increased fuel density and thermal conductivity. The alternative fuel forms all have higher thermal heat conductivities at operational temperatures (approximately 300° C.): uranium nitride—15.5 W/mK; uranium carbide—20.9 W/mK; uranium-zirconium hydrides—18.1 W/mK; and uranium-molybdenum alloys—23 W/mK. Moreover, these materials have thermal conductivities that increase with increasing temperature. Unfortunately, because of the hydrogen production risk, these fuel forms cannot be used in current LWRs.
Another problem facing alternative fuel forms is that they are relatively difficult to sinter. The alternative fuel forms mentioned are refractory materials, that is, they retain their strength even at high temperatures. While this is desirable in the final product, the retained strength requires extraordinarily high temperatures and pressures in the sintering process. Often sintering agents are used, but mixing the powders uniformly is difficult and can require several days.
Some efforts have been undertaken to remedy these problems, but these efforts have failed to provide adequate solutions. One attempt involved encapsulating the entire sintered fuel pellet in a coating. However, a single defect in the coating could lead to catastrophic failure because the whole pellet could oxidize from the single point. Another attempt has been made to supplement the powdered fuel form with additives. In doing so, the advantage of the increased uranium density is lost because the volume of additives necessary to aid sintering is larger than the increase in fuel density achieved by using the alternative fuel form. Finally, attempts have been made to add powders of materials with lower melting temperatures so that those powders will melt and wet each fuel grain during the sintering process. Nevertheless, a complete barrier to corrosion resistance is not created because the liquid metal will not surround every grain barrier.
A need exists in the art to make available alternative nuclear fuel forms so that their superior properties can be utilized. In normal operation, these fuel forms would allow for increased efficiency by allowing for increased heat conduction. In that manner, more energy from each fission reaction can be transferred to the coolant water and thereby to the turbines and generators.